Have do_nothing as default action for dictionary?

[Peter Otten]

Christopher Reimer via Python-list wrote:

> Greetings,
> 
> I was playing around this piece of example code (written from memory).
> 
> 
> def filter_text(key, value):
> 
>  def do_nothing(text): return text
> 
>  return {'this': call_this,
> 
>  'that': call_that,
> 
>  'what': do_nothing
> 
>  }[key](value)
> 
> 
> Is there a way to refactor the code to have the inner do_nothing
> function be the default action for the dictionary?


If it does nothing, why invoke it at all?

LOOKUP = {"this": call_this, ...}

def filter_text(key, value):
    if key in LOOKUP:
        return LOOKUP[key](value)
    return value

If there are much more hits than misses:

def filter_text(key, value):
    try:
        process = return LOOKUP[key]
    except KeyError:
        return value
    return process(value)

If you insist on invoking a noop func:

def do_nothing(text): return text

def filter_text(key, value):
    return LOOKUP.get(key, do_nothing)(value)

With a collections.defaultdict (will grow to comprise new keys):

LOOKUP = defaultdict(LOOKUP, lambda: do_nothing)
def filter_key(key, value):
    return LOOKUP[key](value)


> The original code was a series of if statements. The alternatives
> include using a lambda to replace the inner function or a try-except
> block on the dictionary to return value on KeyError exception.
> 
> What's the most pythonic and fastest?
> 
> Thank you,
> 
> Chris R.
>